This invention relates to an electronic trip unit. More particularly, this invention relates to an electronic trip unit capable of eliminating nuisance tripping at low power in the electronic trip unit.
An electronic trip unit is an intelligent device which controls a circuit breaker. Generally, a microprocessor-controlled electronic trip unit automatically operates the circuit breaker under fault current conditions such as a short-circuit or an overcurrent. The microprocessor-controlled trip unit trips to open the electrical circuit, and therefore, interrupts current flow. The electronic trip unit employs a microcontroller which compares sensed current input with predetermined values and generates a trip pulse when the sensed current exceeds the predetermined values. The generated trip pulse is then used to turn on a MOSFET which energizes a solenoid/flux shifter to trip the circuit breaker.
There are several problems associated with the conventional electronic trip unit. One problem is that the electronic trip unit may exhibit nuisance tripping which can be caused by a malfunction of the microcontroller or glitches from the microcontroller ports at low power. That is, during a self-power mode of the electronic trip unit, when the electronic trip unit voltage is in a range where the microcontroller is in a RESET toggling condition (i.e., when the RESET signal is not stable and changes between high and low values), the noise pick up of the printed circuit board (PCB) track between the microcontroller to the gate of the MOSFET. Therefore, there is a need for an electronic trip unit which filters out microcontroller glitches and PCB noise to ensure that the trip signal is based on a valid fault case.